M. ^ARNOGURSKÁ et al.: HIGH-TEMPERATURE PROCESSING AND RECOVERY OF AUTOCATALYSTS335–340

HIGH-TEMPERATURE PROCESSING AND RECOVERY OFAUTOCATALYSTS

VISOKOTEMPERATURNO PROCESIRANJE IN RECIKLA@AAVTOMOBILSKIH KATALIZATORJEV

Mária ^arnogurská1, Miroslav Pøíhoda2, Marián Lázár1, Peter Kurilla1, Romana Dobáková1

1Technical University of Ko{ice, Faculty of Mechanical Engineering, Vysoko{kolská 4, 04200 Ko{ice, Slovakia2V[B – Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 70833 Ostrava –Poruba, Czech Republic

maria.carnogurska@tuke.sk

Prejem rokopisa – received: 2017-10-12; sprejem za objavo – accepted for publication: 2017-12-19

doi:10.17222/mit.2017.174

This paper presents the results from an experimental study of the thermal method applied for obtaining platinum-group metals(PGMs) by melting used autocatalysts in an 80 kVA plasma reactor, using suitable fluxes and a reducing agent. As the collectorof noble metals, grey cast iron was used during the melting. The melt products were the alloy, inert vitrified slag, syngas and flyash. In the alloy, when melting the catalyser based on cordierite, there was 99.27 % PGMs and 99.00 % in the case ofCr/Ni-strip catalysts. The loss of PGMs in the slag during the melting of cordierite catalysts was 0.34 % and for Cr/Ni catalysts,it was 0.26 %. A chemical analysis of the ash confirmed that in addition to the mechanically removed CaO, SiO2, MgO, Al2O3,it also contained condensed vapours of Fe+PGMs. During the melting of the cordierite catalyst, the ash contained 0.40 % of therefined metals, while 0.74 % was the figure for the Cr/Ni-strip catalyst. The synthesis gas from the process had a very lowheating value (0.16 MJ m–3 or 0.24 MJ m–3).

Keywords: waste, catalyser, platinum-group metals, plasma technology

V tem ~lanku avtorji predstavljajo rezultate eksperimentalnih raziskav termi~nega postopka pridobivanja kovin platinske sku-pine (PGM, angl.: Platinum Group Metals) s taljenjem izrabljenih avtomobilskih katalizatorjev v 80 kVA plazemskem reaktorjuz uporabo primernih talil in redukcijskega sredstva. Kot zbiralnik plemenitih kovin je bila med taljenjem uporabljena siva litina.Produkti taljenja so zlitina, inertna steklasta `lindra, sintezni plin in pepel. V zlitini je bilo pri taljenju katalizatorja na osnovikordierita koncentriranih 99,27 % PGM, pri katalizatorjih s Cr/Ni trakom pa 99,00%. Pri taljenju kordieritnih katalizatorjev seje v `lindri izgubilo 0,34 % PGM, pri Cr/Ni katalizatorjih pa 0,26 %. Kemijska analiza pepela je potrdila, da poleg mehanskopridobljenih oksidov CaO, SiO2, MgO, Al2O3, le-ta vsebuje tudi kondenzirane pare Fe in PGM. V pepelu se je pri taljenjukatalizatorja na osnovi kordierita nahajalo 0,40 % plemenitih kovin, pri katalizatorju s Cr/Ni trakom pa 0,74 %. Sintezni plin izprocesa taljenja je imel zelo nizko kurilno (kalori~no) vrednost (0,16 MJ m–3 oz. 0,24 MJ m–3).

Klju~ne besede: odpadki, katalizator, kovine platinske skupine, plazemska tehnologija

1 INTRODUCTION

Plasma technology for the processing and disposal ofdifferent types of waste is becoming a subject of interestfor many waste producers. This technology can be usedto process, e.g., municipal waste, ash from thermalpower stations, dangerous asbestos-based waste, etc.1–4

Plasma technology is also used when processing variouskinds of industrial waste.5 Noble metals can be recoveredfrom the metallic waste containing them. The recover-ability of noble metals from slag is, for example, thesubject of several papers.6–8 Noble metals can also be ob-tained from slag using chemical-based methods (e.g.,leaching). The basis for this is the change of a givenphase to the liquid phase. The extraction of Cu, Ag, Auand Pd by leaching in nitric acid is described byreferences.9–10 The yield of silver using this method is upto 87 % and for copper, it is 98 %. The leaching of Cu,Zn and Pb from PCBs using electro-generated chlorinein a hydrochloric acid solution is described in refer-ence.11

A separate area in the use of plasma technologies istheir involvement in the process of liquidating catalysers.In this area, results were obtained in various conditionsusing low-temperature thermal plasma. The by-productsfrom processing this waste are always syngas andslag.12–15 The exploitation of the energy from syngas isthe subject of paper16.

The article presents the results of experimental testsperformed using an 80 kVA plasma reactor to melt theautocatalysts. In addition to the autocatalysts, the plasmareactor enables the treatment of waste containing organicand plastic materials (textiles, wood, plastics, etc.) aswell as non-combustible inert materials (glass, ceramics,etc.).

2 DESCRIPTION OF CATALYSERS AND THEIRUSE

Used autocatalysts need to be regenerated and ulti-mately permanently disposed of. In the automotive in-dustry, catalysts are used to reduce the three maincomponents of harmful emissions in the exhaust gases of

Materiali in tehnologije / Materials and technology 52 (2018) 3, 335–340 335

UDK 66.088:628.4.037:628.477.6 ISSN 1580-2949Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)335(2018)

cars – NOx, CO and unburnt hydrocarbons. This meansthat the aforesaid pollutants, with the help of thecatalytic elements (Pt, Pd and Rh) found on the walls ofthe catalyst carrier, are reduced or oxidized to lessharmful chemical compounds.

Platinum and palladium take the predominant sharein the removal of carbon monoxide and hydrocarbons;the rhodium additive is intended to improve the activityof an autocatalyst as reduction reactions lower thecontent of nitrogen oxide. First, platinum with rhodiumcatalyses the splitting of nitrogen oxides into nitrogenand oxygen, and then platinum with palladium catalysesthe oxidation of carbon monoxide (CO) and unburnthydrocarbons with free oxygen into CO2 and water.

3 MATERIAL, TECHNOLOGY, EXPERIMENTALPART

The autocatalysts were melted in the 80 kVA plasmareactor with a dependent electric arc. The plasma-form-ing nitrogen gas was fed into the electric arc through ahollow graphite electrode where it was transformed intoa plasma at a temperature of the order of 103 K. At sucha temperature, independent of the partial pressure ofoxygen, the organic and partly inorganic components ofthe waste are decomposed into simple compoundsaccording to the chemical reaction in Equation (1):11

CaHbOcCldSeNf(s) = xCO(g) + yH2(g) + zN2(g) + wH2O(g) ++ vSO2(g) + rHCl(g) (1)

The non-gaseous components of the waste are meltedand two unmixable liquid phases are formed at thebottom of the plasma reactor – metallic alloy and slag.When melting autocatalysts, the gaseous product is asynthesis gas, which contains two combustible gases(hydrogen and carbon monoxide).

The plasma reactor15 was used to melt autocatalystsbased on cordierite and autocatalysts based on Cr/Nistrips.

A monolithic autocatalyst based on cordierite with aplastic structure (with a circular or elliptical cross-sec-tion) is made of ordinary cordierite with isomorphic iron(2FeO·2Al2O3·5SiO2) or ordinary cordierite with iso-morphic magnesium (2MgO·2Al2O3·5SiO2). The cordi-erite is coated with the �-Al2O3 gel film containing noblemetals (platinum, palladium, iridium, rhodium). Theaverage noble-metal amount in a catalyst is from 1.8 g to2 g per 1 kg of ceramics with a Pt:Rh ratio of 5:1. Thehousing of the autocatalyst, the closure carrier, is madeof stainless steel with a high amount of nickel andchromium (Figure 1).

An autocatalyst on a Cr/Ni strip is made from a chro-mium nickel strip wound into an S shape, on which thereis a coating consisting of a thin film of �-Al2O3 contain-ing Pd, Pt, Rh (Figure 2). Autocatalysts of this type arealso supplied as monolithic carriers (Figure 3) in theform of the plastic of ordinary cordierite with isomorphic

Fe or Mg. The monolithic carriers contain a thin film of�-Al2O3 containing mainly Pd.

Catalysers containing gold, silver, rhenium, rhodium,palladium, iridium or platinum have the waste code 1608 01, while the ADR waste code is B 1150.

Catalysts made of ordinary cordierite with isomor-phic Fe or Mg were ground, before the experiment, to agranularity of < 8 mm. The mix was of a grey colour.Autocatalysts on a Cr/Ni strip were mechanically sepa-rated, before the experiment, from the catalytic layer of�-Al2O3 containing Pd, Pt, Rh with a granularity < 1 mm.The catalysts applied onto a Cr/Ni strip resulted in dustof a yellow-to-cream colour.

Before the experimental tests, the samples were che-mically analysed. The largest components of thecordierite-based autocatalyst were oxides SiO2 (37.03 %mass fraction), Al2O3 (33.20 % mass fraction), MgO(9.80 % mass fraction) and Fe2O3 (1.14 % mass frac-tion). The autocatalyst based on Cr/Ni strips includedAl2O3 (55.67 % mass fraction), CeO2 (12.00 % massfraction), La2O3 (6.87 % mass fraction), SiO2 (2.69 %mass fraction) and ZrO2 (2.59 % mass fraction). In thismelting process, the slag-forming elements are con-sidered to be SiO2, Al2O3, Fe2O3 and MgO or CaO.

The fractions gained in this way, containing noblemetals were subsequently melted in the plasma reactorwith an addition of fluxes (quick lime) and the slag had arelatively low melting point (below 1400 °C). At thistemperature, the slag was saturated with a compound of

M. ^ARNOGURSKÁ et al.: HIGH-TEMPERATURE PROCESSING AND RECOVERY OF AUTOCATALYSTS

336 Materiali in tehnologije / Materials and technology 52 (2018) 3, 335–340

Figure 2: S twisting a Cr/Ni strip into an S shape11

Figure 1: Casing and the cordierite insert11

the xCaO·yAl2O3 type or pure aluminium oxide toprevent corrosion of the lining in the plasma reactor witha high Al2O3 content. When melting the autocatalysts, inaddition to the flux in the plasma reactor, 15 kg of greycast iron was added as a collector of noble metals in theform of cast-iron castings (for each melt). The compo-sition of the charge for both autocatalysts is presented inTable 1. The table also shows the duration of the experi-ment, the average electricity consumption and theconsumption of the nitrogen plasma-forming gas.

The amounts of the noble metals of interest and thechemical compositions of the alloys from the meltingand gasification process for the two autocatalysts aregiven in Table 2. The chemical analysis shows that thealloy of grey cast iron and platinum-group metals is ametal collector. The Fe+PGM alloy is the raw materialfor the hydro-metallurgical production of the noblemetals Pt, Pd and Rh.

The chemical composition of the slag from the melt-ing and gasification of these autocatalysts is summarized

in Table 3. The table shows that all the elements with ahigh affinity to oxygen, such as Al, Si, Mg and Ca, wereconcentrated in the slag.

In the slag, there were also non-sedimentary particlesof Fe-containing noble metals, which represent mecha-nical losses of these metals in the slag (Figure 4). It wastherefore necessary to recycle the slag by crushing,grinding and with the subsequent magnetic separation.

The results of the leaching tests for slag confirmedthat this slag is inert and non-leachable. This means thatno undesirable metals or elements leach from the slagand, therefore, it can be deposited in a landfill as otherwaste. This slag can also be used in civil engineering asa backfill or drainage material. The slag is also suitablefor the production of glass fibres used in the productionof heat- and electrical-insulation materials, as well as inthe metallurgy of iron and steel.

The gas generated in the melting process wasremoved from the plasma reactor through a cyclone

M. ^ARNOGURSKÁ et al.: HIGH-TEMPERATURE PROCESSING AND RECOVERY OF AUTOCATALYSTS

Materiali in tehnologije / Materials and technology 52 (2018) 3, 335–340 337

Table 1: Basic data on the input raw materials for the experiment

Catalyser(kg)

Slag-formingadditive

(kg)

Reduction agent(kg)

Time of experi-ment(min)

Consumption ofel. energy

(kWh)

Consumption ofplasma gas*

(dm3 min–1)cordierit 313.5 47.1 77.5 1863 77.20 8.5

Cr/Ni strips 90.0 77.5 77.5 1175 73.85 8.5*volume of gas media at a pressure of 101.325 kPa and temperature of 25 °C

Table 2: Masses of alloys and their chemical compositions

Fe+PGM alloy(kg)

Chemical compositionPt (g·kg–1) Pd (g·kg–1) Rh (g·kg–1) Fe-C (%) C in Fe (%)

cordierit 16.50 9.47 22.33 6.18 96.04 3.35Cr/Ni strips 18.00 5.095 16.74 3.28 97.21 2.98

Table 3: Masses of slags and their chemical compositions

catalyser slag(kg)

(w/%) � (g)CaO MgO SiO2 Al2O3 Fe+PGM PGM

cordierite 353.50 16.77 12.47 30.08 37.85 1.33 2.12Cr/Ni strips 187.30 42.00 0.33 3.86 47.37 1.02 1.18

Figure 4: Macrophotography of mechanically removed particles ofiron with Pt, Pd and Rh in the slag matrixFigure 3: Carrier of the catalyser11

separator where most of the mechanically removed ashwas trapped. The chemical composition of the fly ashcaptured mainly in the cyclone separator, and sub-sequently in the other filtration devices, is summarized inTable 4. The chemical analysis shows that the ash (apartfrom the oxides of CaO, SiO2, MgO, Al2O3 mechanicallyremoved from the batch and the unreacted carbon addedto the batch) also contained condensed vapour of Fe. Thefly ash can be recycled without any major problems, thusincreasing the share of the precious metals extractedfrom the waste being processed.

Table 4: Mass of fly ash and its chemical analysis

catalyserFlyash(kg)

(v/%) � (g)

CaO MgO SiO2 Al2O3 Ccelk. Fecelk. PGM

cordierit 5.7 16.50 6.55 18.39 13.34 2.47 15.13 2.5Cr/Nipásky 5.9 28.05 3.05 7.92 12.00 2.96 1.40 3.4

5 DISCUSSION OF THE RESULTS OBTAINED

From the point of view of the material and energyutilization of the waste, the plasma melting and gasifi-cation of autocatalysts in the 80 kVA plasma reactor canbe evaluated as follows.

Besides the alloy and slag, a by-product of the melt-ing of autocatalysts is synthesis gas. It results from thereduction of the catalysts and the high-temperature pyro-lytic decomposition of the organic portions of feed-stocks. The gas was removed from the plasma reactor viaa cyclone separator. Most of the mechanically removedash was captured here. Subsequently, the synthesis gaswas cooled from 1500–1600 °C to 160–170 °C in ahydrocyclone and then microparticles were collected in asleeve filter. In the next step, the acidic constituents ofthe synthesis gas were neutralized in a countercurrentabsorption column (pH = 9 of the neutralizing solutionof NaOH). From the absorption column, the synthesisgas was fed into a system of coolers where, in the firststep, it was cooled below the dew point of 60 °C to

precipitate the moisture, with which the synthesis gaswas saturated after the cooling in the hydrocyclone andthe neutralization process. After the cooling, the gas washeated above the dew point and then burnt in a co-generation unit with a CAPSTONE C65 microturbine.

The concentration of the main synthesis-gas compo-nents was continuously measured with a chromatographduring the melting. The results of the measurement ofthe proportions of H2, CO and CO2 in the gas, resultingfrom the melting of the cordierite autocatalyst, is shownin Figure 6. The chromatograph also searched for me-thane but it was not found in the synthesis gas. Offlammable gases, the syngas contained only hydrogenand carbon monoxide. The proportion of H2 fluctuatedbetween 0.79 % and 0.98 % (an average of 0.89 %), andthe CO content ranged from 1.04 % to 1.27 % (an ave-rage of 1.15 %).

The average concentrations of the major componentsof the synthesis gas from the continuous chromatogra-phic measurement of CO, CO2, H2 and N2 during themelting of both types of catalysers are summarised inTable 5.

Table 5: Average composition of syngas

catalyser syngas*

(m3·h–1)(v/%)

H2 CO CO2 N2

cordierite 15.77 0.89 1.14 0.40 97.57Cr/Ni strips 14.91 0.60 0.74 0.28 98.38

*volume of gas media at a pressure of 101.325 kPa and temperature of25 °C

Because of the small proportion of flammable gases,the lower heating value (LHV) of the synthesis gas wasvery low. When processing the cordierite catalyst, the ave-rage LHV was around 0.24 MJ m–3, and only 0.16 MJ m–3

when melting the Cr/Ni-based catalyst. The energybenefit of the synthesis gas with the given LHV wasnegligible in the energy-production process. However, itis appropriate to use synthesis gas in the cogenerationunit.

A division of the platinum-group metals obtainedfrom the alloy, slag and fly ash is shown in Table 6. It is

M. ^ARNOGURSKÁ et al.: HIGH-TEMPERATURE PROCESSING AND RECOVERY OF AUTOCATALYSTS

338 Materiali in tehnologije / Materials and technology 52 (2018) 3, 335–340

Figure 6: Contents of H2, CO and CO2 in the syngas when melting thecordierite autocatalyst

Figure 5: Macrophotography of the slag matrix with a melting auto-catalyst on a Cr/Ni strip

obvious that minimum amounts of these metals passedinto the slag and fly ash. For the Cr/Ni-strip-based auto-catalyst, it was 1.00 % and only 0.74 % for the cordieriteautocatalyst.

Table 6: Fractions of the total PGMs recovered with melt products

catalyseralloy slag fly ash

% PGMcordierite 99.27 0.33 0.40

Cr/Ni strips 99.00 0.26 0.74

The diffusion of metals into the plasma-reactor liningdid not occur because the entire volume of the liquidphases (metal and slag) was kept in the graphite part ofthe reactor. This resulted in a minimum loss of themetals in the lining.

Cast iron was used as the collector of the noble me-tals. It is affordable, has high sorption capabilities withregard to platinum-group metals and magnetic propertiesthat allow mechanically removed metal particles in slagto be separated. This means that the debris from theexperimental melts were magnetically separated aftergrinding and milling, thereby increasing the yield of themetals of interest (Pt, Pd and Rh in Fe).

6 CONCLUSION

The liquidation of used autocatalysts in a plasmareactor can clearly reduce the environmental burden.Three major products can be obtained from the meltingprocess: Fe+PGM alloy, slag and synthesis gas. Thealloy of grey cast iron and platinum-group metals fromthe process of melting is the metal collector. The alloy isa raw material suitable for hydro-metallurgical produc-tion of noble metals Pt, Pd and Rh.

The slag with non-sedimentary Fe particles, whichalso contain noble metals, needs to be recycled to reducethe mechanical loss of metals. After crushing and grind-ing the slag, the iron from the platinum-group metals canbe separated using magnets. The slag can also be used inferrous metallurgy and civil engineering.

The synthesis gas from the melting of both types ofautocatalysts has energy parameters that enable its use inthe production of heat and electricity in a cogenerationunit. The share of the synthesis gas of the averagenatural-gas consumption is less than 0.5 %.

An accompanying feature of melting autocatalysts isfly ash in the synthesis gas that, in addition to the CaO,MgO, SiO2 and Al2O3 oxides removed from the batch,also contains condensed vapour of Fe. It is thereforeadvisable to recycle fly ash, thereby additionally in-creasing the return of the metals of interest from theprocess of melting the catalysts.

Based on the results obtained by melting the auto-catalysts, it can be stated that this waste can be furtherprocessed with the plasma technology into a product ofan inert nature and the resulting melt products. This

means that the presented elimination of the analysedwaste by plasma melting is a waste-free BAT technology.

Previous experimental studies confirm that theprospective areas of use of plasma technologies will bethe recycling and disposal of hazardous waste andlow-level radioactive materials, and the refining ofmaterials to high purity.

Acknowledgements

This research was realised with the support of theKEGA Grant Agency, nos. 003TUKE-4/2016 andSP2017/37-FMMI V[B TUO.

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